I have a 1971 Raleigh International whose Campagnolo Nuovo Record chainset has Campy pedals that are 'frozen' into their threads (I know, this is an instance of neglected maintenance). I really like this bike and its components - pedals included - and would like to have lower gearing than typically possible with a 2-ring chainset (have 42 rear/34 front, now). I'm looking for a master bike machinist who could tap an additional set of 5 holes to accept the 5 sleeves for a triple crankset?
Drilling a crank that old is not a great idea. They weren't that strong to begin with and 40+ years of use hasn't made then any stronger. I would not do anything to further weaken them.
To be honest, if the pedals are frozen I'd think twice about using that crank. Aluminum corrosion expands and can create stress that crack the material ( similar to the way ice can expand and split a crack in a boulder).
All that aside, if you look hard enough you can find a "tripleizer" ring. This is a special inner ring that includes the mounts for a smaller inner ring.
On a bike that old, there's a reasonable chance that you won't need a new bottom bracket to allow the inner ring to clear the chainstays. Also, there's a pretty good chance the existing derailler cage will be low enough to allow use of the small chainring. Back then there was no such thing as a "triple" front derailluer.
Shifting will require a bit of practice compared to modern triples; you will drop the chain, so get a Dog Fang or other chain catcher.
If you want components that are modern but look old and don't cost the insane price of used campagnolo on Ebay; Check out Velo Orange Components This should bring back some memories.
That crank is a 34/48 that I think would be a really good choice for an upgrade on a older bike.
Tap another 5 holes is not enough. There needs to be metal to space it out that I suspect is not there. Just add a spacer would probably not handle the load.
Even if you do attach are you sure there is room on the bike?
Are sure you can find triple front derailleur for that bike?
If you get past all that you may end up with chain line issues.
Look at getting a bigger cassette.